1. Field of the Invention
This invention relates to a cushion element for use between a flexographic printing plate and a printing cylinder during printing.
2. Description of Related Art
Flexography applies broadly to printing processes utilizing flexible substrates bearing elastomeric or rubbery relief printing surfaces. Flexographic printing plates are well known for use in printing, particularly on surfaces which are soft and easily deformable, such as packaging materials, e.g., cardboard, plastic films, etc. Flexographic printing plates can be prepared from photosensitive elements containing photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,749. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. The photosensitive elements generally have a photopolymerizable layer interposed between a support and a coversheet or multilayer cover element. Upon imagewise exposure to actinic radiation, the photopolymerizable layer polymerizes in the exposed areas causing insolubilization of the exposed photopolymerizable composition. Treatment with a suitable solvent removes the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing.
Historically, to mount flexographic plates to a printing cylinder, vinyl sheets having adhesive coated on each side, commonly referred to as stickyback, have been used. Plates are mounted with a partial or entire layer of stickyback between the plate and the printing cylinder. The vinyl sheets are incompressible, thin and tend to vary in caliper. The plate, printing cylinder, gears, substrate and impression cylinder also each have variations in tolerances in surface smoothness and height or thickness. Such inaccuracies dictate the use of increased pressure in the printing process, but such increased pressure causes a deterioration in print quality due to yielding under pressure of the flexographic printing plates. Undesirable results include a dirty appearance of printing and inaccurate reproduction of half tones, e.g., oval dots or halos around characters and images. In addition, there is an increase in the use of thinner plates formed by photopolymerization techniques which can accentuate the resulting problems associated with printing with non-uniform materials such as, plates, cylinders, gears and substrates.
In an effort to overcome the shortcomings of the stickyback sheet, layers of synthetic polymeric foam as backing materials or as tapes are used in mounting the flexographic plate on the printing cylinder. The polymeric foam materials are compressible and thus have sufficient cushioning effect to compensate for the variations in thickness or surface height of the plate, plate cylinder, gears, substrate and impression cylinder. In addition, the foam materials must have sufficient resiliency to rebound rapidly and repeatedly to the original dimensions during printing. However, polymeric foam materials typically fatigue with use during printing since the foam looses compressibility and resiliency, and cannot rebound to its original dimensions.
U.S. Pat. No. 3,285,799 discloses a printing blanket for long periods of use in offset lithography which is composed of a polymeric film and woven backing, an ink transfer layer, and a resilient compressible support layer. The support layer has an external surface subdivided by grooves which leaves flat surfaced islands. The blanket is used as an intermediate to transfer an ink image from a printing plate to paper. The support layer has a durometer of at least 60 Shore A. The support layer contains at least about 0.005 cubic inches of voids per square inch of blanket surface but total void volume does not exceed 40%.
U.S. Pat. No. 5,325,776 discloses a cushioning backing sheet material positioned between a flexographic printing cylinder and a flexible printing plate. The cushioning sheet is an elastomeric material containing widely spaced closed-cell voids which provide pockets within which the encapsulated air can be pneumatically compressed when force is applied, and which will rebound rapidly when the force is relieved. A disadvantage of the closed-cell cushioning material is that the cells break with successive use such that the cushioning material fatigues and looses compression and resilience qualities, and thus print quality deteriorates.